Activation of a variety of cell surface receptors leads the activation of PLC and the subsequent hydrolysis of plasma membrane phospholipid, phosphatidylinositol bis-phosphate (PIP2). The two products of PIP2 hydrolysis, inositol-3-phosphate (IP3) and diacylglycerol (DAG), are second messengers involved in the regulation of cellular Ca2+ and protein kinase C (PKC) activities. PIP2 is also a substrate of a PI-3 kinase. The product of this PI-3 kinase, phosphatidylinositol tris-phosphate (PIP3), appears to be an important second messenger for a distinct cellular pathway. Vertebrate photoreceptor outer segments contain light-sensitive PLC, PKC, and an active PI-3 kinase. In this proposal, the applicant proposes experiments to define the role of PLC and PI-3 kinase. There are 3 specific aims: (1) identification of the light-sensitive PLC in rod outer segments (ROS) and elucidation of its mechanism of activation; (2) investigation of the enzymes in the PI cycle that replenish PIP2 in photoreceptor; and (3) determination of the role of PI-3 kinase in the retina.